docs: JWT allowed skew is not documented

[steve-chavez]

Problem

We allow a 30 second skew when doing JWT exp validation:

postgrest/src/PostgREST/Auth.hs

Line 85 in f3aa00a

allowedSkewSeconds = 30 :: Int64

But this isn't documented anywhere.

[taimoorzaeem]

We also validate iat and nbf claims which are not documented anywhere.

postgrest/src/PostgREST/Auth.hs

Lines 96 to 104 in 3f78615

	failedNbfClaim :: KM.KeyMap JSON.Value -> Bool
	failedNbfClaim mclaims = case KM.lookup "nbf" mclaims of
	Just (JSON.Number secs) -> now < (sciToInt secs - allowedSkewSeconds)
	_ -> False
	failedIatClaim :: KM.KeyMap JSON.Value -> Bool
	failedIatClaim mclaims = case KM.lookup "iat" mclaims of
	Just (JSON.Number secs) -> now < (sciToInt secs - allowedSkewSeconds)
	_ -> False

I think the skew and all claims should be documented in auth.html#jwt-claims-validation.